Terpenes rarely make the nightly news, but these organic hydrocarbon byproducts of eucalyptus, pine, bacteria and even some species of butterflies and termites are emerging as energy-dense players in the energy biojet and biodiesel markets.
Although they are produced naturally in plants and frequently used as fragrances and flavors, terpenes can also be generated from the bioconversion of fermentable sugars derived from lignocellulosic biomass using organisms such as E. coli or S. cerevisiae. In addition, terpenes are produced in a wide number of trees, including pines, terebinth (found in abundance in the Mediterranean) and eucalyptus.
Genetic research may also enable ramped up terpene production in other plant species.
We may be able to move what we learn from eucalyptus to improve terpene production in poplar and switchgrass, according to Gerald Tuskan, a plant geneticist and corporate research fellow at Oak Ridge National Laboratory in Tennessee. In fact, E. coli and S. cerevisiae bacteria can be engineered to produce large quantities of bisabolene, which is a biosynthetic precursor to bisabolane, a biofuel alternative to D2-diesel fuel. Bisabolene makes appropriate blendstocks for jet fuel, as well as other high performance aviation fuels.
Moving genes from eucalyptus or grapes into these bacteria would increase production of this bisabolene to commercial levels, Tuskan said.
And bisabolane also has lower cloud and freezing points than D2-diesel fuel.
Tuskan noted that turning terpenes into biofuel is a catalytic procedure that takes place in large conversion vessels. The biggest technical challenge he said is in identifying and regulating the genes that control the volume and chemical forms of terpenes produced within a plant, such as eucalyptus. Tuskan thinks that other plant species, such as poplar and switchgrass can be modified to increase their own terpene production.
The key to commercial production is in increasing the metabolic efficiency of terpene production levels in microbes to affordable and scalable levels, Tuskan said. Moving genes from eucalyptus or grapes into these bacteria would increase production of bisabolene to commercial levels.
But although bisabolane is similar to D2-diesel, the key to making it viable economically involves production at high yields.
The idea is to achieve high concentrations and yields so that the microbe converts all of the sugar into the fuel as quickly as possible in as small a tank as possible, according to Jay Keasling, professor of chemical engineering at the University of California at Berkeley.
“The goal is not to identify one pathway and optimize it, but to evaluate several, assemble the most powerful, and the optimize it to achieve high titers and productivity,” Blake Simmons, senior manager of the advanced bio-manufacturing group at Sandia National Laboratories, said. Simmons also is the Joint Bioenergy Institute’s (JBEI) chief science and technology officer.
If you could produce terpenes in microbes at the same metabolic efficiency as ethanol, Simmons said, that would have immediate and significant impact in the advanced biofuels community.
Some terpenes, like cineole, a spicy eucalyptus-derived flavoring oil, are ready to be directly converted to biofuel as soon as they are produced, Simmons said. Others, he added, need to be post-processed to meet the fuel specification requirements.
“Recently, we have been using that knowledge to overdrive production in pine trees in a project funded by ARPA-E,” Simmons said, noting that one of the terpenes the researchers are producing is, in fact, bisabolene; which can be reduced to bisabolane via hydrotreatment to remove impurities.
Oxygen is one such impurity.
“If oxygen is present in the terpene, then hydrogenation needs to occur to remove the oxygen,” Gary Peter, a research foundation professor in the School of Forest Resources and Conservation at the University of Florida, said.
Through all these efforts, the hope is that terpenes will eventually see extensive use in a wide-variety of jet fuels. In fact, Peter said, the U.S. Navy holds a patent on a very efficient method of molecular bonding that can result in the production of a very high-density fuel that meets almost all of the specifications for the JP-10 missile fuel.
Tuskan said that the content and concentration of such terpenes varies naturally among various plant species but is predominantly within eucalyptus.
“Amyris, a publicly-traded company that grew out of my laboratory, is the world’s leading company for microbial terpene production,” Keasling said. “They have produced terpene-based diesels that have driven buses in brazil more than 5 million miles and their terpene-based jet fuels have been flown in planes.”
According to Keasling, although Amyris’ fuels are being produced in Brazil, the microbes are engineered in Emeryville, Calif.
Amyris’ (SIP-SPK) jet fuel starts off as the terpene, farnesene (C15H24), produced directly from a microbe. The farnesene is then processed to produce farnesane (C15H32), an acceptable jet fuel blending component.
Simmons said Amyris’ farnesane product has already been demonstrated and validated for use in jet engines, and notes that the commercial roll out of those blended fuels will be within the next two to three years.
As for increasing terpene production in trees themselves?
“Growing sugar-producing plants, extracting the sugars, and getting microbes to produce the fuel is probably more practical than harvesting terpene-producing plants,” Keasling said. “However, we do have a project that is attempting to increase production of terpenes in pine trees.”
Trees also have one other advantage over microbes.
At room temperature, Tuskan said terpenes require specialized membranes to contain or prevent volatilization from a liquid state to a gas. He noted that fungi and bacteria do not have such structures. Through evolutionary time, plants have developed oil glands that can contain (or hold) terpenes in a liquid state.
“Plants by far are better-suited for commercial-scale terpene production right now and into the foreseeable future,” Tuskan said.
The existing pulping and pine chemicals industry are already big players in the emerging terpenes markets, Peter said. He noted that Biofore, part of Finnish paper company UPM, has built and is operating a 30 million-gallon per year hydro-treating facility. Biofore is producing renewable diesel fuel compatible with all engines using pine fatty acids and complex terpenes recovered as crude tall oil in pulp mills.
By some estimates, the bisabolane precursor, bisabolene, could be produced at a final cost of $1.76 per kg, or $5.73 per gal.
“The challenge right now is the price of petroleum fuel; it’s difficult, if not impossible, to compete with fuels produced from $50 per barrel oil,” Keasling said.
However, Simmons sees some daylight for terpene-derived aviation renewables.
“At JBEI, our goal is to produce terpenes at $3 per gallon,” Simmons said.
Lead image: Passenger plane fly down over take-off. Credit: Shutterstock.